Electromagnetic (EM) geophysical exploration methods measure the response of subsurface formations to the propagation of naturally or artificially generated electromagnetic fields. Primary electromagnetic fields may be generated by passing alternating current or pulsing a current through a “transmitter coil” which is an electrically conducting wire or tube which may have an air core or be wrapped around a core made of some electrical conductor. Use of an alternating current is referred to as frequency domain EM while the use of a pulsed current where the current is applied during an on-period and switched off during an off-period is referred to as time-domain EM or transient EM. In both cases, the time-variation of current passing through the transmitter coil produces a response in a large vicinity around the transmitter coil. A transmitter coil may be a small coil made up of many turns of wire or a large loop of wire with multiple turns. Subsurface formations respond to the propagation of time-varying primary electromagnetic fields with the generation of secondary electrical currents by the process of electromagnetic induction (which is the production of a voltage across a conductor when it is exposed to a varying magnetic field) giving rise to secondary electromagnetic fields. The primary and secondary electromagnetic fields may be detected by a “receiver.” A receiver may measure the time-variation of the magnetic field from these currents (for example a coil receiver measuring dB/dt) or may measure the magnetic field itself (a B-field sensor). Hereinafter, the terms “transmitter coil” and “transmitter” may be used interchangeably and the terms “receiver coil” and “receiver” may be used interchangeably.
The primary electromagnetic field travels from the transmitter coil to the receiver coil via paths both above and below the surface of the earth. In the presence of a conducting body or earth material such as soils, rocks, ores or other conducting material, the magnetic component of the electromagnetic field penetrating the subsurface induces time-varying currents, or eddy currents, to flow in the conducting body. The eddy currents generate their own electromagnetic field (referred to as secondary EM field) that travels to the receiver. The receiver then undergoes a response to the resultant of the arriving primary and secondary electromagnetic fields so that the response differs in both phase and amplitude from the response to the primary electromagnetic field alone. Differences between the transmitted and received electromagnetic fields reveal the presence of the conducting body or conducting material and provide information on the conducting body's geometry and electrical properties.
Because electromagnetic fields propagate through air, there is no need for physical contact of either the transmitter coil or receiver coil with the earth's surface. EM surveys can thus proceed much more rapidly than galvanic method surveys, where ground contact is required. More importantly, one or both of transmitter coil and receiver coil can be mounted in or on or towed behind aircraft. Airborne EM methods are used in prospecting for conductive ore bodies and many other geological targets due to their speed and relative cost-effectiveness.
The electromagnetic response from subsurface materials or bodies is dependent on the electrical conductivity of the material or body. Conductive bodies and other structures such as layers with low electrical conductivity exhibit different electromagnetic responses.
Thus, in summary, EM surveying or other geophysical exploration uses the principle of electromagnetic induction to measure the electrical conductivity of the subsurface. In the case of a frequency-domain EM survey, an alternating electric current of known frequency and magnitude is passed through a transmitter coil creating a primary EM field in the space surrounding the coil, including underground. The time-varying EM fields induce a secondary current in underground conductors or structures which results in an alternating secondary magnetic field that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity. The depth of penetration of the EM field into the subsurface is governed by the subsurface electrical conductivity and transmitter excitation frequency and coil separation and orientation.
In the case of a transient EM survey, the same principle of electromagnetic induction is used to measure the electrical conductivity of the subsurface. A pulsed electric current of known amplitude and time-occurrence is passed through a transmitter coil creating a primary EM field in the space surrounding the coil, including underground. The eddy currents generated in the ground in turn induce a time-varying secondary magnetic field that is sensed by the receiving coil. In the off-time of the transmitter, the signal magnitude and time-variation of the signal magnitude is proportional to the terrain conductivity. In the on-time of the transmitter, the received signal is proportional to the terrain conductivity and to the transmitted primary signal. The depth of penetration of the EM field into the subsurface is governed by the terrain conductivity, transmitter power, transmitter excitation frequency and coil orientation.
The depth of penetration of an EM field depends upon its frequency and the electrical conductivity of the medium through which the EM field is propagating. EM fields are attenuated, or weakened, during their passage through the subsurface. The amplitude of the EM field decreases exponentially with depth. The depth of penetration increases as both the frequency of the electromagnetic field and the conductivity of the ground decrease (for example, according to the formula d(m)=503/sqrt(conductivity×frequency)). Consequently, the frequency used in an EM survey can be tuned to a desired depth range in any particular medium.
Accordingly, EM survey systems have traditionally been designed with transmitters to transmit energy in a wide range of frequencies, or be as broadband, as possible. Similarly, receivers employed in EM survey systems are designed to measure the EM response for a wide frequency range typically sacrificing low-frequency sensitivity for high-frequency response. The high-frequency end of the EM spectrum, or range of frequencies, is used to detect subsurface bodies with large electrical resistivity values (and provide near-surface vertical resolution). The low-frequency end of the EM spectrum is used to detect subsurface bodies with low electrical resistivity values (and provide deeper subsurface exploration).
It is impossible, however, with a single system to measure all frequencies of the spectrum well. To generate energy at high frequencies, it is necessary in the time domain to employ a transmitter with a very fast current turn off, which necessitates a low transmitter loop inductance that can be obtained by decreasing the area and number of turns of the wire loop, which also results in a suppressed transmitter dipole moment. The small moment also serves to reduce high-frequency noise by decreasing transients caused by induction of eddy currents within the conductive survey system components and in the aircraft. To further reduce noise in these system transients, other strategies include rigidly connecting the transmitter and receiver, or locating the receiver such that it is minimum-coupled to the transmitter. To perform accurate measurement of high-frequency responses, it is necessary to measure EM responses of very short time duration, which requires a receiver coil with a short self-response which requires a small receiver coil turns-area. These design requirements are in contrast to those for transmitting and measuring low-frequency responses.
To generate low-frequency energy, it is desirable to maximize the transmitter dipole moment by increasing the transmitter turns-area and current passed through the wire loop. However, these design parameters increase the transmitter loop inductance and increase the time for current turn-off (and increasing system transient noise), and are therefore detrimental for transmitting and measuring high-frequency signals. Resonance considerations show that a half-sine waveform is the most efficient choice of waveform for converting electric potential energy into transmitted current. However, this results in a slow current turn-off, which is detrimental to generating high-frequency signals. To measure low-frequency signals, it is desirable to increase the voltage generated in the receiver coil by having a large receiver coil turns-area. However, this increases the coil self-response and therefore decreases ability to measure high-frequency signals. Further, to measure low-frequency signals it is necessary to decrease the effect of receiver motion in the earth's static magnetic field. It is generally desirable to employ soft suspension systems to reduce this effect to the detriment of the measurement of high-frequency signals. These design considerations show that it is impossible to design a single system which transmits and measures high and low-frequency energy well. Thus, a need has arisen for a system that is capable of operating in both low-frequency and high-frequency bandwidths to accurately survey subsurface properties at both shallower depths (for example, near-surface depths) and deeper depths and to detect targets of a wider conductivity range.